The invention relates to switching regulator controller integrated circuits that rely upon sensing the current through switched power devices. In particular, the invention is directed to a circuit that reduces leading edge current spikes in a current sense signal that may occur during switching.
Switching regulator controllers operate by monitoring the amount of current flowing in a switching device such as a transistor driving a transformer. The current is typically monitored by adding a current sense element, such as a resistor or transformer, in series with the switching device. This element in effect transforms the current into a voltage signal which becomes an input to the switching regulator controller. When the level of the sensed input exceeds a set level, the output of the controller integrated circuit will change state. This state change will then disable the switching device which will cause the current in the device to cease.
The current waveform produced by a switching power device usually has a large leading edge spike due to the output rectifier reverse recovery time and transformer parasitics interaction. The current then increases linearly or almost linearly in time until the switching element is disabled. The current signal then decreases to zero.
In current mode control, the output voltage error signal is compared with the peak current. At the point of intersection between the error signal and peak current sensed in the power switch, the output is usually turned off. This way any change in input voltage, output voltage or output current conditions is reflected in the power switch's on time in such a way that the output voltage is kept relatively constant. This is called current mode controlled pulse width modulation (CMC PWM).
Because of the existence of the current spike described above, the sensed current waveform could exceed the error voltage prematurely. If this were to occur, the on time of the switched power device would terminate prematurely. This undesirable condition would cause the power supply to regulate improperly. One existing method for eliminating the leading edge current spike is to place a low pass RC filter between the current sense element and the current sense input of the PWM controller. A filter is formed from passive components that have an attenuation effect over the entire signal that is greater over certain frequency levels than others. As such a filter can insert some distortion into the entire signal, though its primary effect is directed at spike reduction. FIG. 1 shows a sense or current sense signal A with a leading edge spike. The waveform of the current sense signal A after it has been through an RC filter of the prior art is shown as waveform B.
A second method used to avoid the problems of a leading edge spike in a current sense signal is called leading edge blanking. A circuit disables the current sense input for a predetermined time at the beginning of each switching cycle. FIG. 2 illustrates a current sense signal A with a leading edge spike and the output B of a leading edge blanking circuit.